I cut out a few pieces of the slate for the top of the porch & here we are a week later?! I haven't done much since.
I did notice on the new container of Water Putty that you can add a small amount of vinegar or milk when mixing it with the water to slow down the hardening time.
Another thing - I buffed the faux slate with a piece of brown paper bag (it has wax in it), & this created a shine on the higher surfaces, leaving the recessed areas matte. This helped to define the texture of the slate.